Removal of sulfur compounds from wastewater

ABSTRACT

The invention relates to a process for removal of sulfur compounds from wastewater, which process provides recovery of elemental sulfur. The process is carried out in two separate anaerobic reactors. In the first reactor organic compounds are converted to acid compounds and sulfur compounds are converted to sulfide compounds. The sulfide compounds are stripped from said first reactor with a stripping gas. Subsequently, the effluent of the first reactor is fed to a second anaerobic reactor, in which organic compounds, such as the acid compounds, are converted to produce biogas, while any unconverted sulfur compounds from the previous step are converted to sulfide compounds as well. Finally, the sulfide compounds are removed from the second reactor in a stripping system, and converted to elemental sulfur in an adsorber.

FIELD OF THE INVENTION

The invention relates to a process for anaerobic removal of sulfurcompounds from wastewater.

BACKGROUND OF THE INVENTION

Anaerobic biological processes for the treatment of wastewater having ahigh COD (chemical oxygen demand) value are known in the art. In suchknown processes, organic compounds are converted to biogas (a gasmixture comprising CO₂ and CH₄) at temperatures above about 25° C. Whenwastewater contains high amounts of sulfur compounds (e.g., more thanca. 100 mg S/dm³), such as sulfates, problems may occur in operatingsuch anaerobic reactors due to the presence of sulfide, which is formedunder anaerobic conditions from sulfate. Sulfide may inhibit the methaneformation. Also sulfide will give rise to H₂S formation at pH<9. H₂S isa toxic and corrosive gas and requires measures to control odour. Thesulfide may be reoxidized into sulfate, but this requires an additionalaerobic step. Furthermore, regulations often impose a strong limitationon the amount of sulfur compounds that can be discarded into theenvironment.

As a consequence, there is a need for processes to remove sulfide fromwastewater. In the art different approaches have been suggested forsulfide removal processes. For example, EP-A-0 766 650 describes ananaerobic process in which H₂S is stripped from a methanogenic reactorusing a stripping gas. The H₂S rich stripping gas is fed to a scrubberwhere the H₂S is converted into elemental sulfur. The scrubber isoperated with a regenerable redox liquid that contains an iron(III)chelate or an iron(III) complex. During this absorption process sulfideis converted to elemental sulfur, while Fe(III) is reduced to Fe(II).Fe(II) is reoxidized to Fe(III) by air in a separate aerator. This knownprocess is particularly suited for the treatment of alkaline wastewater,such as tannery wastewater.

However, the known processes have a number of drawbacks.

The anaerobic reactor in known processes is operated at a relativelyhigh pH (8-8.5), in particular when alkaline wastewater is treatedhaving a pH of 9-12. As a result, the stripping of H₂S is slow, sinceH₂S dissolves more easily in water at higher pH. To compensate for this,equipment of a large volume has to be employed, in particular thestripper column, which has to be operated with large volumetric flows.An example of such a process employing an external stripper is found inU.S. Pat. No. 5,500,123. Alternatively, acid, such as formic acid, canbe added. Both alternatives bring about high equipment and/or operatingcosts. Another result of operating the anaerobic reactor at a relativelyhigh pH is that carbonate may precipitate in the stripper, which leadsto fouling and clogging of the equipment.

When the H₂S loaded gas is scrubbed in the known processes, inevitablyan amount of CO₂, which is present in the stripped gas as well, isabsorbed in the scrubber liquid. This CO₂ is eventually vented when thescrubber liquid is regenerated. This net removal of CO₂ from the systemgives rise to a further increase of the pH. Moreover, the formation ofcarbonate salts due to the presence of CO₂ in the stripper equipment maygive rise to fouling and clogging. The precipitation of carbonate saltsmay be prevented by lowering the pH, however, this lowers the rate ofabsorption and reaction of H₂S, as a result of which larger absorbersand volumetric flowrates are required.

These drawbacks become apparent for example in the process of EP-A-0 766650, where the aqueous effluent needs to be recirculated over thestripper a number of times in order to lower the sulfide concentration,since per pass only a small amount of sulfide is stripped. This is theresult of the unfavourable equilibrium at higher pH values. Thisrecirculation results in further disadvantages, since the anaerobicreactor is operated at a higher hydraulic loading rate, which may leadto rinsing out of methanogenic sludge from the reactor.

Yet another disadvantage of known processes is that the elemental sulfurthat is formed does not accumulate exclusively in the sulfur settlingtank, but also forms deposits on walls of the tubing, vessels, sprayers,blowers, pumps, packing material, etc., which causes the need forregular cleaning of the equipment.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process whichobviates, at least partly, some or all of the above mentioned problems.The process of the invention thus aims to provide an improvement toanaerobic wastewater treatment with elemental sulfur recovery.

In accordance with the present invention, there is provided a processfor removal of sulfur compounds from wastewater, comprising the stepsof: a) converting in a first anaerobic suspended sludge reactor organiccompounds present in the wastewater to acid compounds, forming aneffluent comprising the acid compounds; b) converting the sulfurcompounds to sulfide compounds in the first reactor, the remainder ofthe sulfur compounds being also comprised by said effluent; c) removingsulfide compounds in said first reactor with a stripping gas, forming agaseous effluent; d) converting the acid compounds in said effluent tobiogas in a second anaerobic reactor; and e) converting sulfur compoundsin said effluent to sulfide compounds in the second anaerobic reactor,forming a second liquid effluent.

It will be understood that by converting the compounds in steps a), b),d) and e), and by stripping in step c) is meant that at least anessential part of the respective compounds present are converted,stripped or removed, the non-converted, non-stripped and non-removedcompounds leaving the reactor in the effluent.

The organic compounds present in the wastewater are converted to acidcompounds, in particular organic (carboxylic) acids, such as (lower)fatty acids. The conversion of organic compounds to acid compounds, aswell as the conversion of sulfur compounds to sulfide is establishedusing microorganisms known to the skilled person. For acidification oforganic compounds bacterial species from the genera Clostridium,Ruminococcus, Propionibacterium, Selenomonas, Micromonospora, the familyof the Lactobacteriaceae, and thermophilic clostridia can be used, aswell as anaerobic sludge in which they are present. For the productionof sulfide from sulfur compounds bacterial species from the generaDesulfovibrio, Desulfotomaculum, Desulfobacter, Desulfococcus,Desulfuromonas, Desulfonema, Desulfobulbus and for thermophilicapplications Sulfolobus can be used, as well as sulfate reducing sludgeor anaerobic sludge in which they are present.

The first (acidification) reactor typically works at 10-40° C.(mesophilic range) or 40-80° C. (thermophilic range), preferably 30-35°C., pressures between 0.3 and 4 bara, mostly 1 bara and biomassconcentrations between 0.1 and 10 g dry weight/dm³, mostly 3 g/dm³. Theratio between stripping gas flow rate and wastewater flow rate can be 3to 100, but is typically 15.

The term ‘suspended sludge reactor’ is used in the present descriptionand claims in its ordinary meaning and encompasses, as the skilledperson knows, reactors in which the sludge is essentially free to movethrough the reactor, viz. reactors in which the sludge is essentiallynot bound to a static surface in the reactor. Such reactors do thereforenot rely on measures to increase the surface area which may serve as asubstrate for the microorganisms, such as packings, etc. The sludge inthese type of reactors is kept in suspension by means of agitation, e.g.by mixing using an agitator, by recirculating the liquids or by bubblinggas through the liquids.

As a result of the production of acid compounds, the pH in the firstreactor will be relatively low. The pH in the first reactor will dependon the type of wastewater to be treated, but is generally maintained ata value lower than 9, preferably lower than 8, most preferably between 6and 7.5. Because of the low pH, the sulfide formed in the first reactoris relatively easily stripped. Moreover, only a relatively small amountof CO₂ is formed. A substantial amount, typically about 50% or more, ofthe sulfur compounds is converted to sulfide, and 50-100% of theproduced sulfide can be absorbed in an absorber. The non-absorbedsulfide is passed to the second reactor, as well as other sulfurcompounds, if present.

Apart from acid compounds, the effluent from the first reactor maycomprise other organic compounds, such as organic compounds that havenot been converted in the first reactor or organic compounds that areintermediates from the reactions in the first reactor. Essentially alltypes of organic compounds may be converted to biogas in the secondanaerobic (methanogenic) reactor. The conversion to biogas, as well asthe conversion of remaining sulfur compounds in this second reactor isestablished using microorganisms known to the skilled person. Thebacterial species that are typically employed for conversion of sulfurcompounds were mentioned above as typical species for converting sulfurin the first reactor. For methane production bacterial species from thegenera Methanococcus, Methanobacterium, Methanocarcina, Methanothrix,Methanophanus and Methanobrevibacter can be used. For thermophilicapplications Methanobacterium thermoautotrophicum or species fromMethanothermus can be used, as well as anaerobic sludge in which theyare present The second (methanogenic) reactor typically works at 10-40°C. (mesophilic range) or 40-80° C. (thermophilic range), preferably30-35° C., pressures between 0.3 and 4 bara, mostly 1 bara and biomassconcentrations between 1 and 50 g dry weight/dm³, mostly 30 g/dm³. Theratio between stripping gas flow rate and wastewater flow rate can be 3to 100, but is typically 20.

The sulfide loaded stripping gas leaves the first reactor and isprocessed in an absorber unit. A suitable absorber unit is described inEP-A-0 766 650 as mentioned herein-above. In detail, H₂S is absorbedfrom a gaseous stream by contacting the gas with a regenerable redoxliquor, having a pH from about 4 to 7. Preferably, the liquor comprisesa transition metal complex, such as an Fe(III) complex, or chelatediron, such as ethylenediamine tetra-acetic acid (EDTA). The metalcomplex is preferably used in a concentration of about 0.01 to about 0.1M.

Alternatively, the absorption of sulfide may be carried out with analkaline scrubber, in which the gas is contacted with an alkalinesolution and the H₂S is converted to alkalihydrosulfide. Thehydrosulfide may be converted to alkalisulfide (e.g. Na₂S), which may bereused in e.g. a tannery process. This type of H₂S absorber isparticularly preferred for tannery wastewater treatment processesaccording to the invention, since Na₂S is used as a raw material intanneries.

The low CO₂ production in the first reactor is very favourable. Sinceonly little CO₂ is produced, the sulfide loaded stripping gas will berelatively low in CO₂ content. As a result, the sulfide may be removedfrom the gas, using means such as a conventional alkaline scrubbingprocess. If CO₂ were to be produced in substantial amounts, this wouldbe removed by the alkaline solution as well. This would result in highalkali consumption in the absorber and in a net removal of CO₂ from thefirst reactor, leading to increased pH values in this reactor. For thisreason, the low production of CO₂ is a considerable advantage of thepresent invention.

Preferably the process of the invention further comprises a step f) forremoving the sulfide compounds from the second effluent (i.e. the liquideffluent from the second reactor) with a stripping system. Again, byremoving in step f) is meant that at least an essential part of thesulfide present is removed, the non-removed sulfide leaving the reactorin the effluent. By removing a substantial amount of sulfide in thefirst reactor, removal of sulfur compounds in the form of sulfide can becarried out more easily in the second reactor, since the concentrationof sulfide in the effluent of the first reactor is already significantlylower. Typically, about 50% of the sulfur present in the wastewater isremoved. Moreover, the methanogenesis process in the second reactor willbe less influenced by the presence of sulfide. The use of two separateanaerobic reactors is thus advantageous, since it is not required toreturn desulfidized water to the methanogenic reactor to decreasesulfide concentration. This prevents (1) a too high hydraulic loadingrate in this reactor and as a consequence (2) risk of sludge wash out,as well as (3) lower methanogenic activities. This is in particularimportant for wastewater types which do not lead to granular sludgeformation in methanogenic reactors, but to the formation of flocculantsludge, e.g. tannery wastewater.

Another important aspect of the present invention is that the sulfide isremoved from the first reactor by feeding a stripping gas through theliquid in the reactor, e.g. by bubbling. This makes the use of anexternal stripper superfluous, thus saving on installation and operationcosts. Also, by using a stripping gas, no H₂S will accumulate in theheadspace of the first reactor, which would occur when the liquid wasfed to an external stripper.

In this respect, mention is made of U.S. Pat. No. 4,614,588, whichdescribes a method for reducing the sulfur content of wastewater byemploying two reactors. Sulfide is removed by exhausting the gas fromthe first reactor. To obtain sufficient separation of H₂S gas (i.e. toproduce sufficient H₂S pressure in the head space of the reactor), it isrequired in the process of U.S. Pat. No. 4,614,588 that the pH is keptbelow 6. Such a requirement poses a strong limitation on the versatilityof the removal process, since alkaline wastewater, e.g. tannerywastewater, cannot suitably be treated in such a process, since it wouldlead to a pH of higher than 6.

Another advantage of flowing stripping gas through the first reactor isthat it enables a good mixing of the water, the anaerobic sludge flocsand the gas, whereby the sludge may be suspended in the water by thegas. This enables the use of the anaerobic contact process (ACP). Thisfeature is specifically constitutes an advantage over the disclosure ofU.S. Pat. No. 4,735,723, which describes a process for the removal ofsulfur compounds from wastewater using two anaerobic reactors. Accordingto this document, a fluidized bed reactor is preferred to carry out theacidification reaction, preferably combined with an external stripper.In fluidized bed reactors a high sludge age is employed. As a result ofthe high sludge age, microorganisms will develop that will convertorganic acids to methane. As a result, the pH of the first reactor willincrease, which is undesirable as was set out above. Sludge ages thatare typical for fluidized bed reactors are generally too high for theprocess of the present invention. The reactor systems suggested in U.S.Pat. No. 4,735,723 separate all water and sludge in the reactor, as aresult of which the sludge remains in the reactor for a period of timethat is generally too high for the process of the present invention.

According to the present invention it is essential that the averagesludge age is kept low, preferably below 5 days, more preferably from1-4 days, most preferably at about 3 days.

This can be obtained by carrying out the separation of water and sludgein an external separator. This can be obtained by using the ACP process,which forms a preferred embodiment of the invention, especially for thetreatment of alkaline wastewater.

In the ACP process wastewater is mixed with recycled sludge solids andthen converted in a reactor sealed off from the entry of air. Thecontents of the reactor are completely mixed and the sludge is presentin the form of suspended flocs. The flocs are kept in suspension byagitation. In the reactor the bacteria in the flocs biologically convertthe compounds in the wastewater to other compounds. After the reaction,the mixed liquor, containing sludge flocs, is transported to a settlingtank in which the flocs settle and the relatively clear supernatant isthe effluent. At the bottom section of the settler a more concentratedsuspension of flocs is formed. A large part of this suspension isrecycled to the reactor, a smaller part is discharged. By settling andrecycling, the formation of flocs by biomass in the system is promoted(by selection). Because of the completely mixed characteristics of thesystem, the sludge retention time (sludge age) can be controlledprecisely by the discharge flow rate of the settled sludge. For example,if every day one-third of the total mass of sludge is removed from thesystem, the sludge age will be 3 days. Since acetoclastic methanogens(which convert acetic acid into methane) need sludge ages of more than 5days (because of their high doubling time), they will not grow in asystem with a sludge age of 3 days.

The ACP process is typically used for wastewater with COD concentrationsof 1500-5000 mg/l and the hydraulic retention time is typically 2-10hours. Although problems have been reported with separation of sludgeand gas in methanogenic anaerobic contact processes, in the presentnon-methanogenic system a better separation is obtained, as the gas isnot produced in the flocs but added in form of bubbles in the waterphase.

Since the methanogenesis from fatty acids is minimized using the ACPprocess, an optimal accumulation of fatty acids and a corresponding lowpH is obtained.

Other types of reactors known in the art are less or not suitable toobtain a suitable sludge age. For example, packed bed reactors, such asanaerobic filter or fixed film reactors, do not enable the control ofsludge age, since in these reactors the biomass (from which the sludgeis formed) is fixed to the packing.

In this respect, reference is made to a publication by E. Särner (Wat.Sci. Tech., 22 (1990) 395-404), which describes a sulphur removalprocess that uses an anaerobic trickling filter (Antric filter as apretreatment step. The pH in this step is kept low by applying arecirculation stream over the filter. The accumulation of organic acidsleads to a pH lower than 6, the minimum pH for methanogenesis. However,such low pH cannot be guaranteed in case alkaline wastewater is treated.Than the high pH and the high sludge age will lead to conversion oforganic acids into methane. In addition, even under low pH conditions,methanogenic activity can occur in the Antric filter; according toSärner, the choice for biofilm processes results in considerable CH₄formation, since it is inevitable that at different biofilm depths,different environmental conditions are found, among which conditionsthat give rise to breakdown of organic acids, whereby CH₄ is produced.In addition, breakdown of organic acids results in a rise of pH, whichmakes this process unsuitable for treatment of alkaline waste water.

Also fluidized bed reactors are less suitable or even unsuitable, sincethese type of reactors are characterized by a considerable spread inresidence time of the biomass. When fluidized support particles, such assand, need to be partly refreshed, a portion of support particlescovered with sludge will remain in the reactor. As the biomass growth onthe fresh sand particles will be much slower than growth on theparticles already covered with biomass, a large spread in sludge agewill develop. As a result a considerable amount of biomass with a veryhigh sludge age will be present in the reactor, which makes thedevelopment of acetoclastic methanogens possible, which results in theconsumption of acetic acid.

Carrying out the desulfurization process in two separate reactorsaccording to the invention, does not affect significantly theinstallation costs of the total plant.

By carrying out the process in accordance with the invention, theproblem of precipitation of carbonate salts in the stripper as used inknown processes is overcome. As a result of the lower pH, a lowerflowrate of the stripping gas can be employed as well as a lowergas/liquid exchanging surface, making the need for an external (packed)stripper, operated with gas as the continuous phase unnecessary.

The biogas that is produced in the second reactor, will contain sulfideas well, which has to be removed. This can be done by conventionalmeans, such as by the method described in EP-A-0 766 650, which wasdescribed herein-above as a means to remove the sulfide from the productgas of the first reactor. By removing the sulfide from the product gas,clean biogas is obtained, which can be used for a variety of purposes,which will be apparent to the skilled person. The sulfide is convertedin the absorber to elemental sulfur. The sulfur containing effluent ofthe absorber is fed through conventional means, such as a coagulator, aflocculator and/or a settling tank to produce a sulfur slurry, whichalso can be used for a variety of purposes, which will be apparent tothe skilled person.

The liquid effluent of the second reactor may still contain aconsiderable amount of sulfide. These sulfide compounds can be removedfrom the liquid effluent, using a stripping column, which is fed with astripping gas comprising CO₂. The stripping gas is brought into contactwith the liquid by any suitable means, such as spraying or trickling theliquid down and flowing the stripping gas in countercurrent. The sulfideloaded stripping gas is then fed to an absorber unit, which is similarto the one described above, to convert the sulfide to elemental sulfur.The remaining desulfidized gas is vented.

Stripping of H₂S from liquids proceeds more easily, i.e. at higher ratesand/or more favorable equilibrium conditions, at relatively low pH (pHabout 6.5 to 7.5) and higher temperatures (about 30-40° C., preferablyat about 31-35, most preferably at about 33° C., instead of about 25°C.). For this reason it is preferred to use a combustion off-gas as thestripping gas in stripping the H₂S from the liquid effluent of thesecond reactor. The CO₂ in the combustion off-gas provides a lower pH tothe liquid, which enhances H₂S stripping and prevents the formation ofcarbonate deposits in the stripper, whereas the heat present in theoff-gas provides a higher temperature for the stripping process.

The combustion gases can be an effluent from conventional combustionprocesses. Another advantage of this embodiment is that no extrameasures have to be taken to obtain CO₂ externally. It is preferred touse biogas produced with the process of the invention for producing theoff-gases, since it can be burned on-site to produce these off-gases.

In addition, it was surprisingly found that, despite the lowersolubility of H₂S in Fe(III)chelate containing liquors, the highertemperatures of the sulfide loaded stripping gas, which results in anincreased temperature in the absorber, the rate of H₂S absorption isenhanced in the absorption step. Also the flocculation of sulfur isenhanced by higher temperatures, as will be discussed below.

In a further preferred embodiment of the invention, the removing of thesulfide in step f) comprises contacting the liquid effluent from thesecond reactor with an oxygen containing gas, preferably air. Thus thestripping of the liquid effluent from the second reactor is carried outusing a gas mixture, which further comprises oxygen. The oxygenconcentration will generally be up to 20 vol. %. Preferably the oxygenconcentration is from 5-20 vol. %, most preferably about 14 vol. %. Tothis end air can be mixed with the stripping gas, such as the off-gasmentioned above. In the process described in EP-A-0 766 650 thereduction of Fe(III) to Fe(II) by sulfide (by which sulfur is produced)and the reoxidation of Fe(II) is carried out by separate processes,i.e., in separate columns. According to EP-A-0 766 650 the oxygenconcentration has to be carefully controlled in order to preventintroduction of oxygen in the system. It is therefore surprising thataccording to the present process it is not required to operate theabsorption of sulfide in the absence of oxygen. The oxygen which isintroduced through the stripping column in the absorber can instead beused to regenerate the Fe(II) to Fe(III) (or another suitable redoxsystem that is used) in the absorber. This makes the presence of aregenerator no longer required in this embodiment. Also means to controlthe oxygen concentration are no longer required, adding to the economicadvantage of this embodiment.

A further preferred embodiment which uses oxygen to regenerate the redoxliquor, comprises feeding the sulfide from the gaseous effluent from thesecond reactor and the sulfide from the liquid effluent from the secondreactor to separate absorbers using the same recirculating redox liquor,in which the sulfide from said gaseous effluent from the second reactoris contacted with said redox liquor in co-current. The stream of biogasis essentially smaller than the gaseous effluent of the stripper used totreat the liquid effluent of the second reactor. Therefore the absorbertreating the biogas can be smaller as well. However, the same absorptionliquid may be used, provided that transfer of oxygen from freshlyregenerated redox liquor to the biogas is prevented. This can beobtained by operating the absorber for the biogas such that the biogasand the regenerated absorption liquid flow in co-current. The oxygenpresent in the absorption liquid thus can react with the reduced metal(e.g. Fe(II) is oxidized to Fe(III)).

In another preferred embodiment, the liquid effluent of the secondreactor is passed over a stripper only once. In conventional processesthe sulfide containing effluent of a methanogenic reactor has to berecirculated over a stripper in order to obtain an acceptable decreasein sulfide concentration. This is also caused by the unfavorablechemical equilibrium as a result of the high pH employed. A disadvantageof recirculation of the aqueous effluent is that the higher hydraulicloading rate of the stripper requires a larger stripper. The secondprerequisite for a smaller stripper is that no recirculation of strippereffluent over the methanogenic reactor is required, which bears thedisadvantage of too high hydraulic loading rates and sludge wash out.According to the present invention, however, recirculation is notrequired, since a substantial amount of sulfide is already removed inthe first reactor (up to about 50%), as a result of which, themethanogenesis in the second reactor is hardly adversely affected. Byusing a suitable stripping gas, such as CO₂ as described above, anoverall sulfur removal of more than 90% can be obtained with the processof the invention, using no recirculation in the stripper for the liquideffluent of the second reactor.

Another preferred embodiment of the process of the invention is any ofthe processes as described above, in which step f) comprises contactingthe sulfide containing biogas effluent and/or a gaseous sulfidecontaining effluent obtained from stripping the liquid effluent of thesecond reactor with a redox liquor at a pH of 7-9, preferably at a pH ofabout 8.5. In the art a pH of 4-7 is said to be optimal for operatingthe absorption step with a redox liquor, in order to minimize absorptionof CO₂ from the sulfide containing gas. If net removal CO₂ from thesystem takes place, it is required to replenish it from external sourcesin order to prevent a pH that would be too high for operating theanaerobic reactor and the stripping process successfully. It was found,however, that the absorption of sulfide is enhanced when a higher pH ofthe redox liquor is employed. When a pH of about 7-9, preferably ofabout 8.5 is used, both the rate of absorption of H₂S and the rate ofconversion to elemental sulfur are increased. When conventionalphosphate buffers are used in this liquor, this will give rise toconsiderable CO₂ losses. However, when bicarbonate buffers are used,preferably in concentration of 0.5-1 M, CO₂ losses can be prevented evenat relatively high gas phase CO₂ concentrations expected (about 10%).

In another preferred embodiment, step f) comprises contacting thesulfide containing biogas effluent and/or a gaseous sulfide containingeffluent obtained from stripping the liquid effluent of the secondreactor with a redox liquor, by which sulfide is converted to sulfur,forming a sulfur comprising effluent, which is fed to a coagulationtank, a flocculation tank and a settler tank, thus forming a sulfur richeffluent. It was found that deposition of solid sulfur on equipment,such as tubing, vessel, gas blowers, pumps and packing material, can beprevented by removing the sulfur as quickly as possible from the redoxliquor. To this end the redox liquid is first fed to the coagulationtank, in which it is stirred vigorously for about 1-3 minutes, givingrise to the formation of small sulfur crystals After the coagulationtank the liquid is fed to a flocculation tank in which the suspension isstirred mildly for 3-10 minutes, by which the crystals will form largerflocs. Flocculation can further be enhanced by the addition ofconventional flocculation agents. The flocs formed can be removed easilyby means of the settler tank to which the liquid is fed next. It wasfurther found that coagulation of sulfur crystals proceeds more rapidlyat elevated temperatures, which constitutes another advantage for theuse of off-gases from combustion processes.

The process of the invention is particularly useful in treatingwastewater with high COD concentrations (>2000 mg/dm³).

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 describes a preferred embodiment of the process of the invention.

This process uses an alkaline scrubber to remove sulfide from thestripping gas in the first reactor. A wastewater influent is fed to afirst (acidifying) reactor. By bubbling stripping gas through the firstreactor the sulfide produced in the first reactor is stripped from thereactor. In the alkaline scrubber the stripping gas is desulfidized bycontacting it with an alkaline solution. This produces a streamcomprising hydrosulfide, which can be used for further processing andclean stripping gas which is recycled to the bottom of the firstreactor. To prevent underpressure in the stripping gas recycle of thefirst reactor (e.g. as a result of absorption of gases in the reactor)or overpressure in this reactor (e.g. as a result of methaneproduction), it may be useful to connect this stripping gas circuit withthe biogas effluent from the second reactor. The liquid effluent fromthe first reactor is passed to a settler tank, in which the sludgesettles and is recirculated to the reactor, while the liquid is fed tothe second (methanogenic) reactor. The liquid effluent of the secondreactor is sprinkled from the top of a stripper column, which is fed atthe bottom with a mixture of combustion off-gas and air. The liquideffluent of this stripping column, which is essentially desulfurized maybe discharged. The biogas produced in the second reactor is passed tothe top of an absorber (Absorber 1) in which it is contacted inco-current with fresh redox liquor. The purified biogas leaves Absorber1 at the bottom. Optionally it can be mixed with hydrogen gas. Thishydrogen gas can be produced by an electrochemical reactor, as isdescribed in EP-A-0 766 650. The stripping gas from the stripper whichis used to desulfidize the liquid effluent of the second reactor is fedto another absorber (Absorber 2). In Absorber 2 the sulfide loadedstripping gas is brought into contact with a fresh redox liquor toconvert sulfide to sulfur. Absorber 2 is operated in countercurrent. Thepurified gas leaving at the top of absorber 2 is vented. The redoxliquors in both absorbers are obtained from the same recirculatingsystem. The bottom products of the absorbers, containing elementalsulfur, are fed to a coagulation tank, a flocculation tank and to asulfur settling tank. The sulfur is obtained as a slurry from the bottomof the settling tank.

EXAMPLES Example 1

An anaerobic reactor of 51 was fed with tannery wastewater with a pH of10. The wastewater contained 3500 mg COD, 50 mg sulfide and 1350 mgsulfate per liter, and was added at a flow rate of 40 l/day. The sludgeresidence time was higher than the hydraulic residence time in thereactor because of sludge sedimentation in the bottom part of thereactor. By daily removing ⅓ of the sludge present, the sludge age wasset at 3 days. In the top of the reactor gas was sparged, after leavingthe reactor the gas was bubbled through a solution of NaOH at a pH of 10and returned to the reactor sparger again. The gas flow rate was 100l/h.

The effluent of the reactor contained 3200 mg COD, 2000 mg VFA (volatilefatty acids), 150 mg sulfate and 30 mg sulfide per liter, and the pH was7.3. The H₂S concentration in the gas leaving the reactor was 6400 ppmand after passing the absorber, the concentration was 3500 ppm. The CO₂concentration in the gas was 1.5% and 1.3% after the absorber.

The results indicate that a large part of the organic compounds areconverted into volatile fatty acids and that because of a lowmethanogenic activity the acids were allowed to accumulate, thus leadingto a low pH and enhancing the stripping process. Simultaneously about90% of the sulfate was reduced to sulfide. This sulfide was removed for90% by stripping and absorption. The alkali scrubber relatively (%)removed more H₂S than CO₂.

Example 2

In a pilot plant H₂S rich gas was produced in a stripper by contactingthe gas with effluent from an anaerobic reactor. This gas was passedthrough a 3 m³ absorber column with a height of 3.5 m, counter-currentwith a redox liquor flow of 5 m³/h. The redox liquor was trickled overthe plastic column packing material and contained Fe(III)EDTA as themain reactant. Its pH was varied using NaOH and CO₂. Two different gasloading rates were tested (50 and 100 m³/h). The temperature was 30° C.The results are summarised in table 1.

TABLE 1 H₂S removal efficiencies in an absorber at various conditionsGas flow pH of H₂S in absorber H₂S H₂S rate redox influent gas inabsorber ef- removal ef- (m³/h) liquor (ppm) fluent gas (ppm) ficiency(%) 50 7.3 25,000 600 97.6 50 7.7 30,000 440 98.5 50 8.0 10,000 175 98.3100 7.5 10,000 775 92.3 100 7.9 9,000 400 95.6 100 8.0 7,000 300 95.7100 8.3 14,000 425 97.0 100 8.4 9,000 300 96.7

A higher pH is beneficial to reach lower effluent H₂S concentrationsand/or higher H₂S removal efficiencies. Because of the higher gasresidence times in the absorber column, higher removal efficiencies wereobtained at lower gas flow rates.

What is claimed is:
 1. Process for removal of sulfur compounds fromwastewater, comprising the steps of: a) converting in a first anaerobicsuspended sludge reactor organic compounds present in the wastewater toacid compounds, forming an effluent comprising the acid compounds; b)converting the sulfur compounds to sulfide compounds in the firstreactor, the remainder of the sulfur compounds being also comprised bysaid effluent; c) removing sulfide compounds in said first reactor witha stripping gas, forming a gaseous effluent; c2) concentrating saideffluent, thus forming a clearer effluent and a more concentratedsuspension of sludge, part of which suspension is recycled to said firstreactor; d) converting the acid compounds in said clearer effluent tobiogas in a second anaerobic reactor; and e) converting sulfur compoundsin said clearer effluent to sulfide compounds in the second anaerobicreactor, forming a second liquid effluent.
 2. Process according to claim1, which further comprises a step f) of removing the sulfide compoundsfrom said second effluent with a stripping system.
 3. Process accordingto claim 2, wherein the removing of the sulfide in step f) comprisescontacting the liquid effluent from the second reactor with a strippinggas comprising CO2, which is an off-gas from a combustion process. 4.Process according to claim 2, wherein step f) comprises stripping theliquid effluent from the second reactor without recycling the liquid. 5.Process according to claim 2, wherein step f) comprises contacting thecontaining biogas effluent and/or a gaseous sulfide containing effluentobtained from stripping the liquid effluent of the second reactor with aredox liquid at a pH of 7-9, which redox liquor comprises a bicarbonatebuffer.
 6. Process according to claim 2, wherein step f) comprisescontacting the sulfide containing biogas effluent and/or a gaseoussulfide containing effluent obtained from stripping the liquid effluentof the second reactor with a redox liquor, by which sulfide is convertedto sulfur, forming a sulfur comprising effluent, which is fed to acoagulation tank, a flocculation tank and a settler tank, thus forming asulfur rich effluent.
 7. Process according to claim 2, wherein theremoving of the sulfide in step f) comprises contacting the liquideffluent from the second reactor with an oxygen containing gas,producing a sulfide containing gaseous effluent and feeding this gasouseffluent to an absorber in which it is contacted with a redox liquor,which absorbs the sulfide.
 8. Process according to claim 7, wherein thesulfide from the biogas effluent from the second reactor and the sulfidestripped from the liquid effluent from the second reactor are absorbedin separate absorbers using the same recirculating redox liquor, andwherein the sulfide from said gaseous effluent from the second reactoris contacted with said redox liquor in co-current.
 9. Process accordingto claim 1, in which the gaseous effluent of step c) is contacted with aredox liquor or an alkaline solution, thereby removing sulfide from thegaseous effluent and producing clean stripping gas which is recirculatedto the process in step c).
 10. Process according to claim 1, wherein thesludge age in the first reactor is from 1-4 days.
 11. Process accordingto claim 1, wherein an anaerobic contact process (ACP) is carried out inthe first reactor.
 12. Process according to claim 1, which furthercomprises: g) contacting the biogas from step d) with a redox liquor,which absorbs sulfide in the redox liquor; h) regenerating the redoxliquor by contacting it with an oxygen containing gas.